1. Field of the Invention
This invention relates to a semiconductor laser device which can attain high reliability even when operated at a high output power level for a long period of time, and more particularly, it relates to a semiconductor laser device which can preferably be used for an optical pickup apparatus. This invention also relates to a simple method for producing such a semiconductor laser device.
2. Description of the Prior Art
A semiconductor laser device which emits laser light from an end facet thereof is a typical example of the semiconductor devices produced by use of the cleavage of semiconductor crystals. A semiconductor laser device of this type has a Fabry-Perot cavity having a pair of end facets and functioning on the basis of a difference in refractive index between the semiconductor crystals and the air outside the device.
In recent years, semiconductor laser devices as described above have widely come into practical use as light sources for various information processing apparatuses such as optical disc driving units and laser printers because they can emit laser light with excellent divergence and high directivity. When semiconductor laser devices are used as the light sources for write-once optical disc driving units or rewritable optical disc driving units, they are required to have high reliability even at a high output power level of about 40 to 50 mW for a long period of time. Furthermore, for the purpose of attaining higher operational speed of an entire system including an optical disc driving unit, there is a demand for semiconductor laser devices which can attain laser oscillation at a still higher output power level. When semiconductor laser devices are used as the light sources for high-resolution laser printers or for optical pumping of solid state laser devices such as a YAG laser, they are required to attain laser oscillation at an output power level of 100 mW or more.
For example, conventional semiconductor laser devices which are prepared from AlGaAs-type semiconductor materials can attain an output power level of about 30 to 40 mW. The higher output power operation of such semiconductor laser devices, however, causes the deterioration of their end facets from which laser light is emitted. The deterioration in the light-emitting facet increases the current required for driving the semiconductor laser device, and eventually it becomes impossible for the laser device to attain laser oscillation. Therefore, with respect to semiconductor laser devices, it is difficult to attain high reliability at a high output power level.
The principal cause for the deterioration of the light-emitting facet is now described. First, heat is generated locally at the light-emitting facet due to the high optical density at this facet and also due to non-radiative recombination caused by the surface state. As the temperature in the area near the facet increases, the band gap in that area becomes smaller, which in turn increases the absorption of light. The increase in the light absorption generates carriers, which are then trapped in the surface state, and non-radiative recombination of the carriers occurs. This further generates heat in the area near the light-emitting facet. This process takes place until the temperature in the area near the facet reaches the melting point of the semiconductor, resulting in facet breakdown.
Moreover, intensive studies to develop semiconductor laser devices which are prepared from InGaAlP-type semiconductor materials and emit red light with a wavelength of 600 nm have been made for the purpose of increasing the density of optical discs or increasing the operational speed of laser printers. In spite of a demand for laser oscillation at a high output power level, the high output power operation of such semiconductor laser devices also causes the deterioration of their end facets from which laser light is emitted (see, e.g., Itaya et al., Preprint of Annual Meeting of the Society of Applied Physics, Spring 1988, 31a-ZP-4,5).
Furthermore, it is desirable that semiconductor laser devices which are used as light sources of optical pickup apparatuses for writing and erasing data on optical discs not only have high output characteristics but also have a small ellipticity .theta..sub.v /.theta..sub.h which is a ratio of the vertical far-field pattern .theta..sub.v to the horizontal far-field pattern .theta..sub.h. Particularly, in the case of a semiconductor laser device having an ellipticity of 2 or less, it is unnecessary to use a beam-reshaping prism for reshaping an elliptical beam emitted from the semiconductor laser device into a circular beam.
When a beam-reshaping prism is used, there is a problem that a light-converging spot is shifted by a change in the wavelength of laser light at the time of writing data on an optical disc. At the time of writing data on an optomagnetic disc, a semiconductor laser device as a light source is used at a high output power level of 30 to 50 mW, and the oscillation spectrum of the semiconductor laser device is in a single mode.
If a light-converging spot is completely focused on the plane of an optical disc by use of a focusing servo system, the wavelength of light emitted from the semiconductor laser device is shifted to the longer wavelength side by the presence of return light from the disc plane. This is because the laser light is allowed to have a gain by the return light, so that the gain distribution peak is shifted to the longer wavelength side by a rate corresponding to the wavelength shift.
In general, beam-reshaping prisms have chromatic aberration, that is, the refractive direction of laser light changes depending upon the wavelength of the laser light. Therefore, if the oscillation wavelength of a semiconductor laser device is shifted by 3 to 10 nm as described above, the light-converging spot is forced to deviate on the plane of an optical disc, and the length of data (bits) written in the optical disc changes, so that it cannot be accurately read out.
It is difficult to avoid the wavelength shift of laser light to the longer wavelength side, because this shift is due to the physical phenomenon that the gain distribution of laser light is changed by return light. Therefore, if no beam-reshaping prism is used, it is possible to prevent the deviation of a light-converging spot. To achieve this prevention, however, the ellipticity of a laser beam emitted from a semiconductor laser device as a light source should be two or less.
To decrease the ellipticity of a laser beam, an attempt has been made to form a waveguide having a width narrower in the cavity portion near the end facets than in the center potion. For example, there can be mentioned a V-channeled substrate inner stripe (VSIS) semiconductor laser device (see, e.g., Appl. Phys. Lett., 40, 372 (1982)), which is produced as follows. First, an n-GaAs layer is grown on a p-GaAs substrate to a thickness of about 1,000 nm and a V-striped channel is formed through the n-GaAs layer in the p-GaAs substrate by photolithography and etching, after which a p-GaAlAs cladding layer filling in the V-striped channel, a p-GaAlAs active layer, an n-GaAlAs cladding layer, and an n-GaAs contact layer are successively grown by liquid phase epitaxy.
The VSIS semiconductor laser device has a loss-guide structure in which light exuding from the active layer is absorbed by the substrate at the shoulder portion of the V-striped channel and this optical loss causes a difference in effective refractive index between the inside and the outside of the V-striped channel. The waveguide width is determined mainly by the width W of the V-striped channel. In a typical case, W=5,000 nm, and the half-value width .theta..sub.h of the horizontal far-field pattern along the junction direction becomes 9.degree. to 10.degree.. Moreover, the half-value width .theta..sub.v of the vertical far-field pattern along the direction perpendicular to the junction direction becomes 23.degree. to 24.degree.. Thus, the ellipticity .theta..sub.v /.theta..sub.h amounts to 2 or more.
Accordingly, an attempt has been made to form a waveguide having a width narrower in the cavity portion near the end facets than in the center portion. For example, if the waveguide width is 3,500 nm at each of the cavity end facets, but it gradually increases with an increase in the distance from the cavity end facet and becomes a uniform width of 5,000 nm in the cavity center portion, the spot of a laser beam is defined by the waveguide width at the light-emitting facet. In a semiconductor laser device having a waveguide 3,500 nm in width at the light-emitting facet, the spot of a laser beam emitted therefrom is narrowed down and the vertical far-field pattern becomes 12.degree., so that the ellipticity can be made two or less.
Because the spot of a laser beam is narrowed down at the light-emitting facet in this way, however, light density is increased and non-radiative recombination centers are formed by oxidation of the light-emitting facet, so that deterioration can readily be caused in the light-emitting facet. Therefore, when the above-described semiconductor laser device is operated at a high output power level of 50 mW or more, there is problem that the reliability thereof may decrease.
In the case of an InGaAlP semiconductor laser device, it is well known that if the end facets thereof are covered with an Al.sub.2 O.sub.3 dielectric film the end facets can be shielded from contact with oxygen so that the end facets can be prevented from degrading. Even in such a semiconductor laser device, however, the dielectric film is formed on the end facets after the formation of the cavity end facets in air by cleavage, and there is oxygen at the interface between the cavity end facet and the dielectric film. Therefore, when the semiconductor laser device is operated to emit laser light, the presence of oxygen causes the deterioration in the end facets.
To prevent the deterioration in the cavity end facets, an improved semiconductor laser device is known, in which a semiconductor crystal having a band gap greater than that of the active layer is grown on the cavity end facets so as to cover the cavity end facets therewith.
In a typical example of the AlGaAs semiconductor laser devices, as shown in FIG. 24, an Al.sub.0.45 Ga.sub.0.55 As cladding layer 82, an Al.sub.0.15 Ga.sub.0.85 As active layer 83, an Al.sub.0.45 Ga.sub.0.55 As cladding layer 84, and a GaAs contact layer 85 are successively grown on a GaAs substrate 81. Laser light is excited within the active layer 83 and emitted from the cavity end facet 80a. On the cavity end facet 80a, an Al.sub.0.4 Ga.sub.0.6 As window layer 86 having an Al mole fraction greater than that of the active layer 83 so as to have a band gap greater than that of the active layer 83.
If such a window layer 86 is grown on the cavity end facet so as to come in contact with the active layer 83, crystal strains are generated in the window layer 86, because the Al mole fraction of the window layer 86 is greater than that of the active layer 83. These crystal strains become crystal defects, and it is therefore impossible to attain high reliability when the semiconductor laser device is operated at a high output power level.
As a method for forming a semiconductor layer having a greater band gap than that of the active layer on the end facets to prevent the deterioration of the end facets, for example, in the case of GaAs/GaAlAs-type semiconductor laser devices, a GaAlAs semiconductor layer having a greater band gap (i.e., having a greater Al mole fraction) than that of the active layer is grown by vapor phase epitaxy on the light-emitting facet formed by cleavage. Such a semiconductor layer is referred to as a large-band-gap layer.
FIGS. 30A to 30H show the conventional production of a V-channel substrate inner stripe (VSIS) semiconductor laser device (see, e.g., Appl. Phys. Lett., 40, 372 (1982)) which is a typical laser for use in optical disc driving units. In particular, FIGS. 30A and 30B show a wafer in which a number of VSIS structures are formed, and one of these VSIS structures, respectively. The VSIS lasers are produced as follows.
First, on a p-GaAs substrate 11, an n-GaAs layer 12 is grown to have a thickness of about 1 .mu.m by liquid phase epitaxy. Then, a V-striped channel 19 reaching the p-GaAs substrate 11 through the n-GaAs layer 12 is formed by photolithography and etching, after which a p-Ga.sub.0.55 Al.sub.0.45 As cladding layer 13 (having a thickness of 0.2 .mu.m outside the V-striped channel 19), a p-Ga.sub.0.88 Al.sub.0.12 As active layer 14 (having a thickness of 0.07 .mu.m), an n-Ga.sub.0.55 Al.sub.0.45 As cladding layer 15 (having a thickness of 1 .mu.m), and an n-GaAs cap layer 16 (having a thickness of 1 .mu.m) are successively grown by liquid phase epitaxy. Laser light exuding from the active layer 14 is absorbed by the shoulder portion of the V-striped channel 19 and this optical loss causes a difference in effective refractive index between the inside and the outside of the V-striped channel, resulting in a loss-guide structure.
The back face of the p-GaAs substrate 11 is rubbed to a wafer thickness of about 100 .mu.m, followed by cleavage of the wafer into bar-shaped wafers 51 (referred hereinafter to as laser bars), as shown in FIG. 30C. In this figure, the illustration of the V-striped channel and other elements shown in FIG. 30A is omitted for simplicity. Thereafter, as shown in FIG. 30D, the respective laser bars 51 are introduced into a preparatory chamber 101 for MOCVD growth, and carried on a susceptor 104 in a growth chamber 103 by the use of a vacuum suction-type pincette 102. On the cleavage plane 52 of the laser bars 51, a Ga.sub.0.5 Al.sub.0.5 As semiconductor layer 53 having a greater band gap than that of the active layer 14 is grown, and the laser bars 51 are carried back in the growth preparatory chamber 101 by the use of the vacuum suction-type pincette 102. Then, the laser bars 51 are taken out of the preparatory chamber 101 (FIG. 30E).
In the laser bar 51 shown in FIG. 30E, the semiconductor layer 53 having a greater band gap than that of the active layer 14 has been grown on the upper face of the n-GaAs cap layer 16, as well as both cleavage planes 52. After removing the semiconductor layer 53 on the n-GaAs cap layer 16 by etching, electrodes 54 and 55 are formed by vacuum deposition on the back face of the p-GaAs substrate 11 and the upper face of the n-GaAs cap layer 16, respectively, as shown in FIG. 30F. Moreover, on the semiconductor layer 53 formed on the cleavage plane 52, an end-facet protective film 57 is formed, as shown in FIGS. 30G and 30H. Finally, the laser bar 51 is separated into chips, and the respective chips are mounted on a heat sink, resulting in a plurality of high-power semiconductor laser devices.
In the conventional production method which has been proposed as described above, however, the process of growing a semiconductor layer having a greater band gap than that of the active layer on the cleavage plane of the laser bar includes the step of carrying the laser bar from the growth preparatory chamber directly into the growth chamber, so that it is necessary to carry the laser bar many times and much effort is needed.
Moreover, the position of the laser bar placed on the susceptor may be deviated from the original position by a turbulent gas flow during the above-described growth, and it is difficult to control the position of the laser bar against the gas flow. Therefore, the thickness of a semiconductor layer grown on the cleavage plane may be varied for each growth, and the thickness distribution may be generated even in the same laser bar. For example, even when the thickness of a semiconductor layer to be grown on the cleavage plane is set to be 0.5 .mu.m, the thickness of the semiconductor layer actually grown has a distribution ranging from 0.01 to 10 .mu.m, and in particular, semiconductor layers having a thickness of 3 .mu.m or less cannot be grown with high controllability. If the semiconductor layer grown on the cleavage plane has a thickness of 3 .mu.m or more, stain in the large-band-gap layer 53 is increased by a difference in the lattice constant between the p-GaAs substrate 11 and the large-band-gap layer 53, so that the large-band-gap layer 53 has poor crystallinity, and in a severe case, large crystal defects are generated in the large-band-gap layer 53.
In such a case, the semiconductor laser device having a large-band-gap layer on the cleavage plane may have poor device characteristics, as compared with a conventional semiconductor laser device having no large-band-gap layer on the cleavage plane. The semiconductor laser device having a large-band-gap layer on the cleavage plane cannot attain sufficient reliability, when it is used alone as a light source or incorporated into a system such as a pickup for optical disc driving units.